Many methods have been used for nanofiltration and desalination, including membrane filtration using polymer thin films with track-etched pores created through bombardment or using aluminum oxide layers produced electrochemically and etched to a desired porosity. Major drawbacks in nanofiltration include the expense of initially creating the nanofilters, the limited use of the membranes and their susceptibility to fouling, as well as the energy (i.e., heating, cooling, and/or water pressure) requirements of the process.
Thin film composite membranes allow optimization of nanofiltration through careful selection of the permeability of each individual layer through varying the membrane layer material. However, increased fluid pressure due to flow through the thin film composite membranes tend to compact the varying material layers, thus altering their carefully chosen porosity, undermining the molecular size cutoff for the filtration module.
Filtering aqueous solutions and other water-based fluids for purposes, such as dialysis and seawater purification, for example, could be greatly enhanced if a precise filtration cutoff could be easily tuned. Many filters and other separation devices have an allowable threshold (i.e., filtration cutoff) for the size of molecules able to pass through the filter without being caught or otherwise stopped. This threshold can have a wide range depending on the filter type.
On the nanoscale level, most conventional filtration is limited to membrane filtration, which uses particularly sized pores to limit the molecules passing through the membrane. The specific filtration application both guides and limits the selection and design of the porosity and filtration cutoff for the membranes. The filtration cutoff for membrane filters may only be as precise as the pore creation process allows. For example, membranes created through interfacial polymerization are inexpensive, but have a wider than desirable pore size distribution. Then, once the pores are created in the membrane, their size is fixed and cannot easily be adjusted. Further, the membranes themselves may be susceptible to leaks or tears over time, which destroys the initial filtration cutoff.
Therefore, in order to provide greater precision and flexibility in nanofiltration and desalination systems, new membrane designs are needed, which include sharper filtration cutoffs and modularity for multiple applications. Research on the permeability of graphene oxide films has shown that graphene oxide laminates do not readily permeate ions and molecules with hydrated radii beyond an acceptable range.